The present invention relates generally to navigational systems and more specifically to a transdigitizer as used in a sonobuoy relay for relaying signals from U.S. Air Force Global Positioning System (GPS) satellites.
The Soviet Union uses operational launch control centers to test launch their inter continental ballistic missile systems (ICBM's). The United States test launches its ICBM's from Vandenberg AFB, Calif. where missiles are launched into the South Pacific near the Kwajalein Missile Range or one of several broad ocean area (BOA) target areas. Although the missiles have their own internal navigation systems, one of the purposes of the test launches is to determine the accuracy of the missile systems.
Therefore, testing of ballistic missile systems requires that the splash point of reentry bodies be accurately determined. This determination is usually made through use of the Sonobuoy Missile Impact Location system (SMILS). With this system up to 15 sonobuoys are dropped in an array in the target area. Each buoy has the ability of detecting acoustic (in water) noise generated by the splash of the reentry body and of transmitting the reentry data to an aircraft circling overhead. It is essential that the geodetic position of each sonobuoy be known. The system is used for determining the position of the buoys through use of the signals from the GPS satellites.
The buoys are dropped by the mission support aircraft. As the buoy falls free from the aircraft a small drag parachute deploys which stabilizes the decent of the buoy into the water. At the time of impact with the water the parachute is released, and an antenna is erected. In some buoy types the antenna is located in a small balloon (float) assembly which is inflated by gas from a pressure bottle. The balloon serves to provide extra flotation for the buoy as well as protection for the antenna from salt spray. When the antenna is deployed, the buoy releases a hydrophone assembly which descends to a specified depth. In the case of buoys that are used with this system the typical depth is 30 feet. The hydrophone picks up acoustic energy from the water and transmits this data to the aircraft circling overhead via a VHF radio link. Many buoy types also have the capability of generating acoustic pings (pulses).
Several different types of buoys are used in the GPS SMILS system. The velocimeter buoy is used to measure the velocity of sound in water. A bathythermograph buoy may also be deployed to measure water temperature as a function of depth.
Three of the buoys in the array are known as GPS transdigitizer buoys (GTD). These buoys, developed by the Applied Physics Laboratory of the Johns Hopkins University, receive signals from the GPS satellites in view and translate them down to base band where they are one-bit quantized and sampled at a 2 megasample/sec rate. These data along with a data stream generated by digitizing the acoustic data are then used to modulate a carrier for transmission to an aircraft that is circling overhead. The GTD buoy is also equipped with a command receiver for turning the transdigitizer power off and on and for commanding acoustic pings.
In addition to the three GTD buoys and velocimeter buoy, additional buoys are used in the array that are similar to the standard ASW (Anti-Submarine Warfare) buoy. These buoys receive acoustic data, relay it to the supporting aircraft, and also generate acoustic pings. The availability of more than three buoys reduces the possibility of signals being missed because of a particular buoy receiving simultaneous signals from more than one reentry body splash. A large number of buoys also provides improved geometry which results in improved accuracy in the determination of the splash position.
The primary function of the GTD buoys is to provide the position and orientation of the buoy array. As described previously, signals received at the buoy from satellites in view are digitized and transmitted to the supporting aircraft. The data are both used in real time and recorded for post-test analysis in the support aircraft. Special Digital Tracker Channels (DTC) track the signals from each of the satellites received by each buoy. The output of these tracker channels are pseudoranges and pseudorange rates. By processing thes measurements from three of four satellites (limited by the availability of DTC) the position of the GTD buoys in the water is accurately determined.
To determine the position of the other buoys the acoustic pings generated by each of the buoys are received by the other buoys, and are transmitted to the aircraft. These data are processed in the aircraft to provide the ranges between the buoys, which determine the relative position of the buoys. Because the positions of the GTD buoys are known, the geodetic position of each of the buoys can then be determined.
When a reentry body strikes the water it will create a large sonic pulse. The time of receipt of this pulse at each of the buoys can be accurately measured. This information is then used to determine the position of the actual splashes.
In view of the foregoing discussion, it is apparent that the testing of ballistic missiles requires not only instrumentation to determine the position of the missile as a function of time during the missile flight but also a means for determining the impact point of each reentry body. The present invention uses signals from the Air Force's Global Positioning System (GPS) satellites to determine the geodetic position of each sonabuoy which, in turn, permits the generation of test data which will accurately determine the location of the impact points of each reentry body.